1. Field of the Invention
The present invention relates to an electromagnetic bandgap structure, and more particularly to an electromagnetic bandgap structure capable of blocking noise of a specific frequency band and a printed circuit board having the same.
2. Description of the Related Art
With the recent rapid advancement of technologies and services relating to cable and radio broadcasting and communication, the strength of users' demands for products is heightened. In order to realize the reduction in size of products for the sake of portability, an increase in battery performance and multifunctionality of products (which are required to satisfy the above demands), operating speed and bandwidth of electronic products are rapidly being increased. With the increase in operating speed, a clock frequency falls within a GHz range, and thus problems concerned with power integrity (PI), signal integrity (SI) and EMI caused by Simultaneous Switching Noise (SSN) generated from various on/off chips or packages such as digital blocks positioned on a multilayered PCB are emerging as critical issues in PCB design.
SSN, which is also known as a delta-I noise or ground bounce noise, is a source of serious noise on a multilayered PCB. The SSN is caused by time-varying current that rapidly varies in a high-speed digital circuit. SSN, which is generated between a power layer and a ground layer, affects an adjacent signal line, thus not only exerting an influence on signal integrity (SI) but also causing radiation of electromagnetic waves at a peripheral region of a PCB.
More specifically, when a lot of switching devices are switched concurrently, high-speed current passing through vias between multilayered PCBS causes propagation of an undesired electromagnetic wave through a substrate. Since the generated electromagnetic waves may affect transmission characteristics of a signal passing through vias, the electromagnetic waves may serve as a noise source affecting other devices and may increase EMI radiated from a peripheral region of a PCB to the outside.
FIG. 1 is a schematic cross-sectional view of a printed circuit board 10 on which two different electronic circuits 30 and 50 are mounted. The printed circuit board 10 includes power and ground layers 11 and 13 for supplying power to the two electronic circuits 30 and 50, signal lines 17 and a via 15 for signal transmission between the signal lines 17.
As shown in the drawing, when the electronic circuits 30 and 50 operating at different frequencies are mounted on a printed circuit board, electromagnetic waves caused by an operating frequency and harmonic components of one of the two electronic circuits are transmitted to the other electronic circuit and interfere with electromagnetic waves originating from the other electronic circuit, thus causing noise.
The most typical solution to solve the problems of influencing PI and SI and EMI, which occur as a result of the SSN generated from a high-speed digital system, is to connect a decoupling capacitor between the power layer and the ground layer. In addition, since a feedback current pathway having low impedance is provided through the decoupling capacitor, noise propagated through parallel conductive plates can be blocked. However, in order to reduce SSN for a lot of vias, there is a need for a lot of decoupling capacitors, which increases both production costs and an area of a PCB occupied by the decoupling capacitors, thus restricting the free arrangement of other elements on the PCB. Furthermore, parasitic impedance of the decoupling capacitors causes another problem of parallel resonance. Therefore, the decoupling capacitors are not so efficient in reducing noise occurring from a PCB in a high-frequency band of 1 GHz or more which is causing to trouble for recent high-speed digital systems.
For this reason, research on an electromagnetic bandgap (EBG) structure which is capable of selecting a frequency so as to solve the problems of SSN occurring in the GHz band is being intensively conducted. The electromagnetic bandgap structure cannot only solve the problem of SSN using the characteristic in which high impedance occurs in a specific frequency band but can also solve problems such as the increased cost resulting from use of the decoupling capacitors and the restriction in free arrangement of elements.
Among EBGs, a mushroom type EBG structure and a planar type EBG structure are typically known in the art. However, the conventional EBG structures have disadvantages in that they are hard to design so as to have a stop-band at a desired size which is small enough to be incorporated in a compact electronic device such as a cellular phone.
Accordingly, in response to the above problems, development of an EBG structure which can be reduced in size is eagerly required.